Method for separating and purifying amino acid

This application is a continuation of application Ser. No. 371,312, filed Apr. 23, 1982, now abandoned.

This invention relates to a method for separating and purifying an amino acid and more particularly to a method for separating and purifying an amino acid by piezo-dialysis using an amphoteric ion exchange membrane comprising a domain which has a cation exchange group, a domain which has an anion exchange group and another domain which has no ion exchange group.

There have been developed various methods for separating and purifying an amino acid from an amino acid fermented broth containing impurities such as protein, an organic acid, an inorganic salt, a saccharide and a pigment. In most of such conventional methods, an ion-exchange resin is used in combination with a process of crystallization. However, since large quantities of acid, alkali, water, etc. are consumed therein, the conventional methods have presented problems relative to the production cost and waste liquid treatment. Besides, these methods have been rather ineffective for desalting the amino acid. Meanwhile, there has also been developed a method in which electro-dialysis is carried out with a cation exchange membrane and an anion exchange membrane. However, this method necessitates a large electric power consumption.

Heretofore, compound membranes have been studied for use as amphoteric ion exchange membranes. Such studies include a compound membrane which is prepared by embedding beads having cation exchange groups and beads having anion exchange groups alternately in a neutral membrane made of a silicone resin or the like and a compound membrane which is prepared by embedding an anion exchange resin in a cation exchange resin. However, these membranes are not only difficult to prepare but also do not have domains chemically bonding therein. Therefore, their strength is insufficient for piezo-dialysis. The present inventors have discovered that a membrane made from a ternary block copolymer of molecular structure in which a macromolecule having a cation exchange group, a macromolecule having an anion exchange group and a macromolecule having no ion exchange group are bonded together in a straight chain like state is capable of allowing a low molecular weight electrolyte to permeate it through piezo-dialysis; and that, by utilizing this property of the membrane, an amino acid can be separated and purified. The above stated ternary block copolymer, in the form of a film, forms a micro-phase separated structure which consists of three phases of a size close to the molecular size thereof. As a result of this, a domain having a cation exchange group and a domain having an anion exchange group exist separately from each other. Accordingly, the film functions as amphoteric ion exchange membrane.

It is known that generally a blend of a macromolecule having a cation exchange group and a macromolecule having an anion exchange group forms a neutral complex called a poly-ion complex. Partial or perfect salt formation similar to this takes place also in the case of a binary block copolymer consisting of a cation exchange group and an anion exchange group. Then, the film of the binary block copolymer does not function well as ion exchanger. Whereas, the film according to the present invention is made from a ternary block copolymer containing a neutral segment. With the film material thus arranged, a domain having a cation exchange group and a domain having an anion exchange group are separated from each other and are thus prevented from forming a poly-ion complex.

The method of the invention hereinafter will be described in further detail. In accordance with the method of the present invention, amino acids are separated from each other and/or an amino acid is purified by carrying out piezo-dialysis with a compound membrane made from a ternary block copolymer consisting of segments in which a constituent has a cation exchange group and another constituent has an anion exchange group while a remaining constituent has no ion exchange group. In carrying out the invented method, an amino acid containing solution to be used as crude material has an amino acid concentration between 0.1 g/dl and 30 g/dl. Pressure to be applied is between 10 kg/cm 2 and 150 kg/cm 2 and is preferably 30 kg/cm 2 and more. The pressure must be sufficient for overcoming osmotic pressure acting between the two faces of the film and is within a range permissible by the dialyzing apparatus and the strength of the film. Therefore, the pressure required varies with the concentration of the impurities such as inorganic salts, etc. contained in the crude material solution.

To carry out the piezo-dialysis, the pH of the crude material solution is determined at a suitable value within a range from 2 to 13 according to the kind of the amino acid to be separated and purified. At the isoelectric point of the amino acid, the electric charge of the amino acid apparently becomes zero. Therefore, when the dialysis is carried out at the isoelectric point, a low molecular weight electrolyte which is contained as an impurity in an amino acid solution or an amino acid which is not at an isoelectric point can be selectively allowed to permeate while an amino acid which is at an isoelectric point can be prevented from permeating. Accordingly, the invented method is not only effective for removing impurities contained in the amino acid such as protein, an organic acid, a saccharide, an inorganic salt and organic matters but also effective for separating amino acids of different isoelectric points from each other.

To desalt an amino acid, piezo-dialysis is carried out at a pH adjusted to the isoelectric point of the amino acid. Further, to separate a desired amino acid from an amino acid fermented broth, piezo-dialysis is first carried out at the isoelectric point of the amino acid to remove a low molecular weight electrolyte and then piezo-dialysis is again carried out by adjusting the pH to a region other than the isoelectric point and vice versa.

Among the segments constituting the ternary block copolymer to be used in accordance with the invented method, the constituent segment which forms a domain having a cation exchange group may be selected from polymers of unsaturated carboxylic acid esters or cyano group containing monomers such as acrylonitrile, styrene or α-methyl styrene. The constituent segment which forms a domain having an anion exchange group may be selected from polymers of compounds having heterocyclic rings containing nitrogen atoms such as vinyl pyridine, vinyl pyrimidine, or vinyl quinoline and polymers of styrene derived amines such as a vnyl benzyl dialkyl amine. These polymers permit introduction of a cation exchange group or an anion exchange group without difficulty by known methods such as methods of carrying out hydrolysis, sulfonation, quaternization, etc. The remaining constituent segment which forms a domain having no ion exchange group is preferably selected from diene polymers such as butadiene, isoprene, etc. The strength of the film can be increased by crosslinking this segment. Further, the amount of this neutral domain contained in the ternary block copolymer is preferably between 30 and 90%. With regard to the ternary block copolymer, the present applicant previously filed a U.S. patent application Ser. No. 206,562 filed Nov. 13, 1980, now U.S. Pat. No. 4,365,023, under the tile of "Block Copolymer."

Assuming that the segment which is capable of introducing a cation exchange group is expressed as "poly A," the segment which is capable of introducing an anion exchange group as "poly B" and the segment which is not introducing any ion exchange group as "poly C," since dialysis is to be carried out under pressure, the blocks of the block copolymer to be used for a membrane for separating and purifying an amino acid are preferably arranged to have the cation segment and the anion segment separated from each other by the neutral segment in such a manner as poly A-poly C-poly B; poly C-poly A-poly C-poly B; poly A-poly C-poly B-poly C; or poly C-poly A-poly C-poly B-poly C. Arrangement to have the cation segment and the anion segment adjacent to each other in such a manner as poly A-poly B-poly C tends to impair the separating efficiency of the membrane. Meanwhile, even if each of the cation, anion and neutral segments is a copolymer consisting of two or more kinds of monomers, it would make no difference in the results obtainable in accordance with the invention.

The invention will more fully be understood from the following description of embodiment examples. However, it is to be understood that the present invention is not limited to these examples. Futher, it is apparent from the results obtained in accordance with the invention that the amphoteric ion exchange membrane which is composed of the ternary block copolymer is not only applicable to amino acids but also readily usable for separation of a low molecular weight electrolyte and a low molecular weight non-electrolyte from each other in general.